The present invention relates to an improvement in a thermally insulating container or the Dewar container. In particular, the invention relates to a thermally insulating container useful for storage and transportation of low-temperature liquefied gases, having double walls with an evacuated space in between, which is improved in the preservation of thermal insulation properties over a prolonged duration of storage or transportation.
In the prior art, thermally insulating containers for storage and transportation of low-temperature liquefied gases, such as, liquid nitrogen, liquid oxygen, liquid helium, liquid argon, liquid hydrogen, liquefied natural gas and the like, have two or more walls made usually of a metal with the spaces between the walls evacuated to a highly reduced pressure. These metal vacuum containers have been intended to prevent heat transfer from outside by means of vacuum thermal insulation and reduce the evaporation loss of a liquefied gas contained therein. In order to further reduce such heat transfer, it has been proposed to provide multi-laminar thermal insulators made of material having a low heat conductivity within the evacuated space, or on the surfaces facing the vacuum.
The degree of vacuum in the thermally insulating container is required to be as high as possible to obtain the most excellent thermal insulation, while it is a general trend that such vacuum thermal insulation gradually decreases in the long run. The reason for the decreasing tendency in the thermal insulation is supposed to be an increase in pressure within the vacuum due to release from the inside the walls of traces of certain gases inherently existing within the metal constituting the walls. Therefore, attempts have been made to prevent such released gases from being isolated and accumulated within the vacuum, but successful and satisfactory results have not been achieved.